1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus comprising at least a charging unit, an image exposure unit, a reversal development unit, an image transfer unit, and an electrophotographic photoconductor.
2. Discussion of Background
An image forming apparatus such as a printer, a copying machine or a facsimile machine can produce an image through a series of steps of charging, image exposure, development and image transfer. Therefore, such an image forming apparatus comprises at least a charging unit, an image exposure unit, a development unit (a reversal development unit in the present invention), an image transfer unit, and an electrophotographic photoconductor.
The above-mentioned image forming apparatus has the drawback that an abnormal image often occurs while continuously operated for an extended period of time. To eliminate such a drawback of the image forming apparatus, there are some proposals with respect to the electrophotographic photoconductor placed in the image forming apparatus. Such conventional proposals are as follows:
(1) Japanese Laid-Open Patent Application 11-15181 (MINOLTA Co., Ltd.)
An electrophotographic photoconductor is fabricated in such a manner that the surface of an aluminum or aluminum alloy support is subjected to anodizing, followed by mechanical abrasive finishing and sealing. On the support which has been subjected to sealing by dipping the support in hot water or putting the support in a moistening system, a photoconductive layer is provided.
(2) Japanese Laid-Open Patent Application 10-301314 (MINOLTA Co., Ltd.)
An electrophotographic photoconductor comprises an electroconductive support, an undercoat layer formed thereon, and a photoconductive layer formed on the undercoat layer. The aforementioned undercoat layer comprises a composition of an organoalkoxysiloxane and colloidal alumina, which composition is cured by the application of heat thereto.
(3) Japanese Laid-Open Patent Application 10-90931 (MINOLTA Co., Ltd.)
An electrophotographic photoconductor comprises an electroconductive support, an undercoat layer formed thereon, and a photoconductive layer formed on the undercoat layer. The aforementioned undercoat layer comprises a resin and heat-treated titanium oxide.
(4) Japanese Laid-Open Patent Application 5-204181 (KONICA CORPORATION)
An electrophotographic photoconductor comprises a support, and an electroconductive polyaniline layer and a photoconductive layer which are successively overlaid on the support in this order.
(5) Japanese Laid-Open Patent Application 8-44096 (Ricoh Company, Ltd.)
An electrophotographic photoconductor comprises an electroconductive support, an undercoat layer formed on the support comprising titanium oxide and a thermosetting resin, and a photoconductive layer formed on the undercoat layer. The amount ratio by volume of the thermosetting resin for use in the undercoat layer is controlled to 0.5 to 0.6 vol. %, and the average particle size of the titanium oxide particles for use in the undercoat layer is adjusted to 0.4 .mu.m or less. Further, there is disclosed an image forming apparatus employing the above-mentioned electrophotographic photoconductor and a reversal development unit.
(6) Japanese Laid-Open Patent Application 9-34152 (KONICA CORPORATION)
An electrophotographic photoconductor comprises an electroconductive support comprising aluminum, aluminum-manganese alloy, aluminum-magnesium alloy or aluminum-magnesium-silica alloy, an undercoat layer which is formed on the electroconductive support and comprises a compound selected from the group consisting of a metal alkoxide, an organic metal chelate, a silane coupling agent and reaction products thereof, and a photoconductive layer formed on the undercoat layer.
(7) Japanese Laid-Open Patent Application 9-292730 (KONICA CORPORATION)
An electrophotographic photoconductor for use with reversal development, comprises an electroconductive support comprising aluminum or an aluminum alloy, and an anodized layer and a photoconductive layer which are successively overlaid on the electroconductive support in this order. The distance (Sm) between the adjacent convex portions on the surface of the anodized layer is controlled to 0.3 to 250 .mu.m, and the maximum height (Rt) of the convex portion is 0.5 to 2.5 .mu.m. Further, the surface glossiness of the anodized layer is controlled to 60 gloss or more. (8) Japanese Laid-Open Patent Application 10-83093 (Ricoh Company, Ltd.) An electrophotographic photoconductor comprises an electroconductive support, and an undercoat layer and a photoconductive layer which are successively overlaid on the electroconductive support. The undercoat layer comprises finely-divided particles of titanium oxide, with the surface portions of the titanium oxide particles comprising at least zirconium oxide. (9) Japanese Laid-Open Patent Application 5-11473 (KONICA CORPORATION)
An electrophotographic photoconductor comprises a cylindrical electroconductive support and a photoconductive layer formed thereon. On the outer surface of the cylindrical electroconductive support, a plurality of grooves are arranged in a row around the circumference of the cylindrical support, each groove having a width of 10 .mu.m to 1 mm and a depth of 0.1 to 5 .mu.m and the section of each groove in the direction of the width thereof being regular. In addition, the photoconductive layer comprises as a charge generation material crystals of a mixture of a specific titanyl phthalocyanine and vanadyl phthalocyanine.
(10) Japanese Laid-Open Patent Application 6-54745 (KONICA CORPORATION)
There is disclosed reversal development method using a photoconductor which comprises a specific titanyl phthalocyanine and a specific hydrazone compound.
(11) Japanese Laid-Open Patent Application 10-221871 (KONICA CORPORATION)
There is disclosed a method of forming an image, comprising the steps of charging an electrophotographic photoconductor comprising a specific titanyl phthalocyanine to a predetermined polarity, forming a latent electrostatic image on the photoconductor using a light emitting diode (LED) as the light source, and developing the latent electrostatic image to a visible image by reversal development.
(12) Japanese Laid-Open Patent Application 7-152184 (Matsushita Electric Industrial Co., Ltd.)
An electrophotographic photoconductor comprises an electroconductive support and a layered photoconductive layer formed thereon. The photoconductive layer comprises a charge generation layer and a charge transport layer, which are successively overlaid on the electroconductive support in this order. The charge transport layer formation liquid comprises 1,1-bis(p-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene as a charge transport material and tetrahydrofuran as a solvent.
In the previously mentioned proposals (1) through (9), an undercoat layer comprising a specific material is provided between the electroconductive support and the photoconductive layer, or the anodized film is deposited on the surface of the electroconductive support in order to prevent the injection of the hole into the photoconductive layer or the charge generation layer from the electroconductive support in the course of reversal development. Namely, the object is to prevent the toner deposition of the background of the photoconductor.
However, the above-mentioned various materials for use in the undercoat layer and the provision of the anodized film on the electroconductive support have a serious effect on the electrostatic properties of the photoconductor under the circumstances of high temperature and high humidity and low temperature and low humidity. For instance, the sensitivity of the photoconductor is lowered, and the potential of an image portion (a light-exposed portion) on the photoconductor is increased after the repeated operation. Thus, the image density of the obtained toner image tends to decrease.
The object of each of the previously mentioned proposals (9) to (11) is to provide a photoconductor capable of minimizing the toner deposition on the background and showing stable characteristics in the continuous operation of reversal development by employing a specific titanyl phthalocyanine alone or in combination with a specific charge transport material.
When the titanyl phthalocyanine pigment is used as a charge generation material, the sensitivity of the obtained photoconductor can be increased. This is because the titanyl phthalocyanine pigment for use in the charge generation layer can generate a large number of charge carriers and the charge carriers thus generated can be readily injected into the charge transport layer. However, since the barrier properties of such a charge generation layer itself is extremely poor, a defective image will appear promptly if the hole is injected into the charge generation layer from the electroconductive support. In addition, local defects present in the charge transport layer and the undercoat layer cannot be compensated.
According to the proposal (12), the coating liquid for the formation of the charge transport layer comprises 1,1-bis(p-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene as a charge transport material and tetrahydrofuran as a solvent. The partial deterioration of charging characteristics of the photoconductor is considered to be caused by the remaining solvent component such as dichloromethane in the charge transport layer. Further, when the remaining solvent is removed from the charge transport layer by drying the charge transport layer formation liquid for a long period of time, cracks tend to occur in the obtained charge transport layer, thereby causing the noise of the produced image. Therefore, tetrahydrofuran is chosen as the solvent for the charge transport layer formation liquid in this proposal. As a matter of course, the occurrence of toner deposition on the background can be reduced in the reversal development by this proposal. However, in such a photoconductor, the toner deposition on the background is caused by the increase in residual potential and the increase in the potential of the light-exposed portion due to the deterioration of the photosensitivity during the continuous operation. Therefore, the decrease in the image density of the light-exposed portion, that is, the image portion is inevitable.